Distributed amplifiers are commonly employed for ultra-broad bandwidth applications. The amplifiers achieve a broad bandwidth by absorbing parasitic elements of transistors into lowpass artificial input and output transmission lines with shunt capacitances mainly provided by the transistors and series inductors added by a designer. Common designs typically have a transistor per “section” and the amplified current from each section adds in phase at the output of the amplifiers.
Referring to FIG. 1, a diagram of a conventional uniform distributed amplifier 90 is shown. Distributions in the amplifier 90 are uniform with series inductances, shunt capacitances, and transistors of uniform size in each section. Termination resistors are used on both the input lines and the output lines to flatten gain and present favorable impedances. On the input side, a bias voltage is often applied to the input transmission line through the input termination resistor because the current criterion is low and so a negligible voltage drop is created through the resistor. A conventional bias injection method on the output side uses large inductive bias chokes on the output line—either integrated, external, or a combination of both. The conventional methods are effective, but external inductors are expensive and occupy very large circuit board area in cases where the amplifier extends to low frequencies. With uniform distributed amplifiers, the bias voltage on the output transmission line can also be applied through the output termination resistor if the current is relatively low, but cannot be implemented where no output line termination resistor is available.
It would be desirable to implement distributed transconductance amplifiers.